JP4292068B2 - Scanning electron microscope - Google Patents

Description

  The present invention relates to a scanning electron microscope, and more particularly to the configuration and control of the secondary electron detection system.

  A scanning electron microscope is an apparatus that forms a sample image by scanning and irradiating a sample with a primary electron beam, and detecting secondary electrons generated from the sample in synchronization with scanning by a secondary electron detector. Usually, secondary electrons having very small energy of several eV to several tens eV are generated from the sample irradiated with the primary electron beam, and the generated secondary electrons are applied from the secondary electron detector. It is guided to a 10 kV lateral positive electric field and taken into the secondary electron detector. In addition to this basic process, in recent years, a bias electrode to which a positive voltage of several hundred volts is applied in the vicinity of a sample is provided to correct the trajectory of secondary electrons to the detector side to increase the yield. The bias electrode is a plate-like or net-like one called “secondary electron collector electrode” or “auxiliary electrode”, and seeks an optimum value by changing the shape or applied voltage to increase the secondary electron yield. A number of electrodes that aim to increase the yield of secondary electrons using these positive electric fields have been proposed so far, and it was common to "orbitally correct secondary electrons with negative charges". (Japanese Patent Laid-Open No. 6-103951). On the other hand, in Japanese Patent Laid-Open No. 9-147782, a ring-shaped electrode to which a negative voltage of −100 V to −500 V is applied is arranged on the upper part of the sample (U-shaped in the direction opposite to the secondary electron detector). A method for guiding secondary electrons in the direction of the detector is described.

JP-A-6-103951 Japanese Patent Laid-Open No. 9-147782

  Secondary electrons generated from the sample by primary electron beam irradiation are usually emitted in various directions in the range of 0 to 90 degrees in the vertical direction and 0 to 360 degrees in the horizontal direction. In JP-A-9-147782, a U-shaped negative voltage application electrode opened in the direction of the secondary electron detector is disposed, and secondary electrons having various directions as described above are arranged as secondary electron detectors. It is guiding in the direction of. However, when the secondary electron trajectory is analyzed by simulation, the secondary electrons not only collide with and are absorbed by the lower part of the objective lens or the lower structure, but also various detectors installed near the sample stage and other objective lenses ( It was found that only a part of the generated secondary electrons reached the secondary electron detector as a result. This tendency is remarkable when high-resolution observation is performed, that is, when the objective lens is close to the sample, and as a result, only an image with poor image quality is obtained.

  The present invention positively corrects the trajectory of secondary electrons emitted from the sample (especially secondary electrons passing near the optical axis) to improve the detection efficiency of secondary electrons by the secondary electron detector. Objective.

  In the present invention, the secondary electrons emitted from the sample in the direction close to the optical axis are positively corrected to the secondary electron detector side, and the secondary electrons are detected while being placed on the avoidance trajectory with respect to the ground potential component. Guide to the vessel. For this purpose, the first auxiliary electrode is disposed near the objective lens lower part (lower magnetic pole) or the primary electron beam irradiation port of the objective lens lower structure, and a negative potential of 1 to 30 V is applied to the lateral potential. By strengthening, the trajectory of the secondary electrons generated from the sample is corrected to the secondary electron detector side as much as possible.

  That is, the present invention includes a sample stage that holds a sample, an objective lens that irradiates the sample with a converged primary electron beam, and a secondary electron detector that detects secondary electrons generated from the sample by electron beam irradiation. In the scanning electron microscope, the first auxiliary electrode having a negative potential is provided near the primary electron beam irradiation port below the objective lens. The first auxiliary electrode forms a lateral electric field directed in the outer circumferential direction, and this electric field deflects the trajectory of the secondary electrons generated from the sample in the outer circumferential direction and enters the secondary electron detector arranged in the lateral direction 2. It works to increase secondary electrons.

  It is preferable to install a second auxiliary electrode having a positive potential on at least the secondary electron detector side of the first auxiliary electrode. The second auxiliary electrode deflects secondary electrons pushed back to the sample side by the negative electric field of the first auxiliary electrode toward the secondary electron detector, and enters the secondary electron detector arranged in the lateral direction. Acts to increase electrons.

  The first auxiliary electrode has a ring shape or a U-shape that is open on the side facing the secondary detector, and the second auxiliary electrode is suitable for a ring shape or a secondary electron detector having a larger diameter than the first auxiliary electrode. It can be made into a U-shaped shape with the closed side closed. The potential of the first auxiliary electrode is preferably about −5 to −20V, and the potential of the second auxiliary electrode is preferably about +5 to + 20V.

  More preferably, a third auxiliary electrode having a positive potential is provided between the secondary electron detector and the sample. Specifically, the secondary electron detector is composed of a scintillator (electron-to-light conversion element) in which a positive voltage of about 10 kV is applied to the surface and a photomultiplier tube for detecting the converted light, and the scintillator surface electrode A plate-like third auxiliary electrode is placed between the sample and a positive potential is applied. Thereby, the secondary electron trajectory is deflected and guided in the direction of the scintillator, and the secondary electrons incident on the secondary electron detector can be increased.

  The present invention also provides a sample stage for holding a sample, an objective lens for irradiating the sample with a converged primary electron beam, a secondary electron detector for detecting secondary electrons generated from the sample by electron beam irradiation, and The scanning electron microscope includes an annular backscattered electron detector having a primary electron beam passage hole below the objective lens, and the backscattered electron detector has a first auxiliary electrode having a negative potential near the primary electron beam passage hole. A second auxiliary electrode having a positive potential on the outer peripheral side. The potential of the first auxiliary electrode is preferably about −5 to −20V, and the potential of the second auxiliary electrode is preferably about +5 to + 20V.

  According to the present invention, secondary electrons generated from a sample can be efficiently taken into a secondary electron detector, and an image with high image quality can be obtained.

  Embodiments of the present invention will be described below with reference to the drawings. In order to facilitate understanding, the same components are denoted by the same reference numerals in the following drawings.

  FIG. 1 is a schematic diagram of a conventional scanning electron microscope, and is a diagram for explaining the trajectory of secondary electrons generated from a sample by irradiation with a primary electron beam. The sample 2 is held on the sample stage 20 in the sample chamber 18 evacuated. When the primary electron beam 3 irradiated from the lower surface of the objective lens 1 is scanned on the surface of the sample 2, secondary electrons 4 and 6 having information on the sample 2 are generated. The secondary electrons 4 are secondary electrons emitted at a small angle with respect to the optical axis, and the secondary electrons 6 are secondary electrons emitted at a large angle with respect to the optical axis. A sample image is formed by taking the secondary electrons into a secondary electron detector (a positive electric field of about 10 kV is formed in the vicinity of the scintillator) 5 attached in a certain direction. However, there are components that become obstacles (ground potential) that change the secondary electron trajectory before reaching the secondary electron detector 5, which hinders the efficiency of secondary electron detection. This is because secondary electrons with a negative charge can be drawn at a positive potential like a secondary electron detector, but at the same time, if there is a ground potential near the orbit where secondary electrons are drawn, This occurs by bending the equipotential surface and changing the secondary electron trajectory itself. Examples of components having a ground potential that hinder the efficiency of secondary electron detection include an analytical instrument 19 such as EDX, WDX, and EBSP, a sample fine movement stage 20, and the lower surface of the objective lens 1 facing the sample 2. .

  FIG. 2 shows the energy distribution of secondary electrons in a general scanning electron microscope, and FIG. 3 shows the direction of generation of secondary electrons generated from the sample. As shown in FIG. 2, the secondary electrons generated from the sample have energy of several eV to several tens eV. The reflected electrons have almost the same energy as the primary electron beam. Usually, from the sample irradiated with primary electrons, the direction perpendicular to the optical axis is ± 90 degrees as shown in FIG. 3A, and the horizontal direction is shown in FIG. 3B. The secondary electrons 7 of several eV to several tens eV are emitted in various directions at 360 degrees.

  FIG. 4 shows a secondary electron orbit by simulation. FIG. 4A is a layout diagram of the objective lens 1, the sample 2, and the secondary electron detector 5 that are the premise of the simulation. A bias electrode 12 to which a positive voltage of 300 V was applied was provided between the sample 2 and the secondary electron detector 5. A positive voltage of 10 kV is applied to the secondary electron detector 5 in order to draw secondary electrons. FIG. 4B assumes the generation direction as shown in FIG. 3 as the generation direction of secondary electrons from the sample, and 2 when assuming that secondary electrons of about several eV to several tens eV are generated from the sample. It is a simulation result of the next electron orbit.

  4B, among secondary electrons having an energy width of several eV to several tens eV generated from the sample, secondary electrons on the relatively low energy side of about several eV are generated by the secondary electron detector. Although it is drawn to some extent by the positive electric field, some secondary electrons on the high energy side of about several tens of eV collide with the objective lens, and some are not affected by the drawn electric field generated by the secondary electron detector. It can be seen that this causes a decrease in detection efficiency. Further, it can be visually seen that secondary electrons in the vicinity of the optical axis are difficult to be drawn to the secondary electron detector side. The present invention pays attention to the problems in the present apparatus and aims to improve it.

  FIG. 5 is a schematic view of a scanning electron microscope according to the present invention. When the primary electron beam 3 irradiated from the lower surface of the objective lens 1 is scanned on the surface of the sample 2, secondary electrons 4 and 6 having sample information are generated. The secondary electrons 4 and 6 are taken into a secondary electron detector (a positive electric field of about 10 kV is formed in the vicinity of the scintillator) to form an image. In the present invention, in order to positively correct the trajectory of the secondary electrons 4 radiated in the direction near the optical axis, the first auxiliary is located near the irradiation port of the primary electron beam 3 of the lower structure of the objective lens 1. The electrode 13 is installed. The shape of the first auxiliary electrode 13 is preferably a ring shape that surrounds the optical axis or a U-shape that is open on the side facing the secondary electron detector 5. A negative voltage of minus several volts to minus several tens of volts (about -5 to -20V) is applied to the first auxiliary electrode 13, and the trajectory is corrected so that secondary electrons do not collide with an object having a ground potential such as the lower magnetic pole of the objective lens. To do.

  The reason why the ring-shaped or U-shaped shape with the open side facing the secondary electron detector 5 is good is that the secondary electrons generated from the sample are generated in all directions as shown in FIG. is there. In the case of a U-shape, if the U-shaped open part is installed so as to face the direction of the secondary electron detector, it becomes possible to give directionality to secondary electrons generated in all directions. Is also effective. Further, regarding the voltage applied to the first auxiliary electrode 13, if a negative voltage equal to or higher than the energy of the negatively charged secondary electrons is applied, generation of secondary electrons from the sample itself is suppressed. As can be seen from the energy distribution of the secondary electrons shown in FIG. 2, the energy width of the secondary electrons is about 5 to 20 eV, so there is no need to apply a negative voltage beyond this. Further, in a general scanning electron microscope, the distance (working distance) between the sample and the lower surface of the objective lens can be arbitrarily changed, so that the negative electric field applied to the secondary electrons is the same even if the working distance is changed. The negative voltage applied to the first auxiliary electrode 13 can be arbitrarily changed in the range of about −5 to −20V.

  More preferably, the second auxiliary electrode 14 is disposed on the secondary electron detector side of the first auxiliary electrode 13 so that a positive potential of several volts (about +5 to +20 V) can be applied. The second auxiliary electrode 14 serves to guide the secondary electrons as much as possible to the secondary electron detector side by pulling up the secondary electron trajectory pushed back to the sample side by the first auxiliary electrode 13 again. The shape of the second auxiliary electrode 14 may be a ring shape or a U-shape like the first auxiliary electrode. When it is U-shaped, it is installed with the closed side of the U-shape facing the secondary electron detector.

  The reason why a positive potential of several volts (about +5 to +20 V) is applied to the second auxiliary electrode 14 is that the voltage applied by the first auxiliary electrode 13 is minus several volts (about −5 to −20 V). This is because it is not necessary to apply a reverse bias equal to or higher than the voltage applied by the second auxiliary electrode 14. For example, when a negative voltage of −Va volts is applied to the first auxiliary electrode 13 and a positive voltage of + Vb (Vb> Va) is applied to the second auxiliary electrode 14, the secondary electrons are caused by the electric field generated by the second auxiliary electrode 14. This is because the secondary electrons are strongly influenced, and secondary electrons are drawn (collision) into the second auxiliary electrode itself, and as a result, cannot be drawn in the direction of the secondary electron detector 5. Also, since the working distance can be arbitrarily changed in a general scanning electron microscope, the second auxiliary electrode is always the same as the first auxiliary electrode 13 so that the positive electric field applied to the secondary electrons by the second auxiliary electrode is the same. The applied voltage of the electrode can be arbitrarily changed in the range of about +5 to + 20V.

  In order to guide the secondary electrons whose trajectory has been corrected by the first auxiliary electrode 13 and the second auxiliary electrode 14 to the secondary electron detector 5, the electrode of the secondary electron detector 5 is disposed in front of the electrode of the secondary electron detector 5. A third auxiliary electrode having a structure facing the sample side in an insulated state is arranged, and a positive potential of several tens to several hundreds of volts is applied. This third auxiliary electrode is the same as the bias electrode installed in the conventional scanning electron microscope. Since the conventional scanning electron microscope is provided with only the third auxiliary electrode, the secondary electron detection efficiency cannot be increased as described above. However, secondary electron detection efficiency can be improved by combining the first auxiliary electrode, the second auxiliary electrode, and the third auxiliary electrode according to the present invention.

  By installing the third auxiliary electrode 12, it is possible to correct the trajectory of secondary electrons emitted to the space between the lower part of the objective lens 1 and the secondary electron detector 5. This avoids obstacles (ground potential) near the lower magnetic pole of the objective lens, such as the detector (EDX / WDX / EBSP) 19 around the sample stage, sample fine movement stage 20, and other samples. It plays the role of guiding in the direction of the secondary electron detector 5. As with the first auxiliary electrode 13 and the second auxiliary electrode 14, even if the working distance is changed, the voltage applied to the third auxiliary electrode can be arbitrarily changed so that the electric field applied to the secondary electrons is always the same. To do.

  FIG. 6 is a diagram showing a simulation result of the secondary electron trajectory when the first auxiliary electrode 13, the second auxiliary electrode 14, and the third auxiliary electrode 12 are installed. Comparing FIG. 4B and FIG. 6, it can be seen that the orbit lines of the secondary electrons are increased in the portion A shown by being circled in FIG. Further, when attention is paid to the portion B surrounded by a circle in FIG. 6, in the case of the present invention, it can be seen that the secondary electrons are guided toward the secondary electron detector without colliding with the lower surface of the objective lens 1. . The effect of installing the auxiliary electrode is more effective when the first auxiliary electrode 13 and the second auxiliary electrode 14 are installed at the same time than when only the first auxiliary electrode 13 is installed. If the second auxiliary electrode 14 and the third auxiliary electrode 12 are installed simultaneously, the effect is further increased.

  In the present invention, the repulsive force and the positive electric field generated between the negatively charged secondary electron and the electrode applied to the negative potential are not simply guided to the secondary electron detector direction using the positive electric field. The secondary electron yield is increased by moderately controlling the attractive force due to. As a result, it is possible to increase the yield of secondary electrons as compared to the conventional case, and it is possible to observe the primary electron beam to be irradiated as narrowed as possible. By increasing the detection efficiency of secondary electrons, observation with the lens condition set to a higher resolution condition becomes possible, which contributes to improvement in observation resolution and image quality.

  FIG. 7 is a schematic diagram showing the configuration of a scanning electron microscope using a semiconductor backscattered electron detector. When the primary electron beam 3 irradiated from the lower surface of the objective lens is scanned on the sample surface, reflected electrons 22 having the composition information of the sample are generated together with the secondary electrons 6. As shown in FIG. 2, the reflected electrons have a higher energy than the secondary electrons and have an energy equivalent to that of the primary electron beam. The semiconductor backscattered electron detector 15 is installed immediately below the lower magnetic pole of the objective lens as a detector for the purpose of observing a backscattered electron image. The problem is that the semiconductor backscattered electron detector 15 is also at ground potential, which adversely affects the secondary electron trajectory. Depending on the sample, it may be desired to observe the secondary electron image using the secondary electron detector 5 at the same time as the reflected electron image observation with the semiconductor backscattered electron detector 15 (while installing the backscattered electron detector). . In this case, if no countermeasure is taken, the secondary electron trajectory is affected by the ground potential by the semiconductor backscattered electron detector 15 and consequently the secondary electron detection efficiency is lowered. Therefore, in the present invention, the first auxiliary electrode 13 or the first auxiliary electrode 13 and the second auxiliary electrode 14 are incorporated into the semiconductor detector 15.

  8A and 8B are diagrams showing a structure example of the semiconductor backscattered electron detector according to the present invention. FIG. 8A is a cross-sectional view and FIG. 8B is a bottom view. This semiconductor detector 15 is obtained by forming a four-divided semiconductor detection element 16 on a semiconductor detection element base 17. In the center, there is a hole through which the primary electron beam passes. In the present invention, a conducting wire or an electrode capable of generating an electric field is previously incorporated in the semiconductor element portion of the semiconductor detector 15. An electrode to be the first auxiliary electrode 13 is installed on the inner diameter side, and an electrode to be the second auxiliary electrode 14 is incorporated on the outer diameter side. By using the semiconductor detector 15 having this structure, it becomes possible to simultaneously capture reflected electrons with the semiconductor detector without impairing the yield of secondary electrons by the secondary electron detector 5. The problem of observation can be solved. The reflected electrons have substantially the same energy as that of the primary electron beam applied to the sample, and the normal acceleration voltage is 0.5 kV to 30 kV. Therefore, the first auxiliary electrode having a negative potential is used for detecting the reflected electrons. 13 has almost no effect.

  FIG. 9 is a diagram showing an example in which the scanning electron microscope according to the present invention is applied to low vacuum secondary electron detection. The mechanism of low-vacuum secondary electron detection (gas amplification type detection method) is that secondary electrons emitted from a sample (normally secondary electrons in a low-vacuum state) when the degree of vacuum in the sample chamber is 1 to about 3000 Pa. It is difficult to observe) by pulling up with a positive electric field and accelerating it, and amplifying positive ions with secondary electron information by repeating collisions with residual gas molecules like an avalanche phenomenon. Thereafter, the positive ions are taken from the sample into the amplifier as a sample current (absorption current) to form a low vacuum secondary electron image.

  According to the scanning electron microscope of the present invention, a positive voltage used for detecting the low vacuum secondary electrons can be applied from the first auxiliary electrode 13 and the second auxiliary electrode 14. That is, by controlling the voltage applied to the first auxiliary electrode 13 and the second auxiliary electrode 14 so that both electrodes are set to a positive potential, the sample chamber pressure is maintained at 1 Pa or higher and the sample is irradiated with an electron beam to generate ions. It can also be used as an electrode for applying an electric field for a gas amplification type detection system that detects an image and forms an image. In this case, it is advantageous that these auxiliary electrodes can be controlled to be negative or positive depending on the vacuum state of the sample chamber. When a semiconductor detector including the first auxiliary electrode 13 and the second auxiliary electrode 14 is attached, a composition image, a stereoscopic image, and the like of reflected electrons and a gas-amplified secondary electron image by the semiconductor detector can be observed simultaneously.

The figure which shows the structure of a general scanning electron microscope. The energy distribution map of the secondary electron generated from a sample. The figure which shows the generation direction of the secondary electron which generate | occur | produces from a sample. The figure which shows the position and secondary electron orbit of the secondary electron detector in a common scanning electron microscope. The figure which shows the structural example of the scanning electron microscope by this invention. The figure which shows the difference of the secondary electron orbit of the scanning electron microscope of this invention, and a general scanning electron microscope. The figure which shows the other structural example of the scanning electron microscope by this invention. The figure which shows the structural example of the semiconductor backscattered electron detector by this invention. The figure which shows the example which applied the scanning electron microscope by this invention to the low vacuum secondary electron detection.

Explanation of symbols

  1: objective lens, 2: sample, 3: primary electron beam, 4: orbit of secondary electron, 5: secondary electron detector, 6: orbit of secondary electron, 12: third auxiliary electrode, 13: first 1 auxiliary electrode, 14: second auxiliary electrode, 15: semiconductor detector, 16: semiconductor detection element, 17: semiconductor detection element base, 18: sample chamber, 19: detector (EDX / WDX / EBSP), 20: stage , 22: Reflected electrons

Claims (7)

A sample stage for holding the sample;
An objective lens for irradiating the sample with the converged primary electron beam;
In a scanning electron microscope including a secondary electron detector for detecting secondary electrons generated from a sample by electron beam irradiation,
A first auxiliary electrode disposed under the objective lens and applied with a negative voltage;
A positive voltage is applied so as to form an electric field that pulls up the secondary electrons pushed back to the sample side by the first auxiliary electrode, and between the first auxiliary electrode and the secondary electron detector. A distance from the optical axis of the primary electron beam farther than the first auxiliary electrode, and a second auxiliary electrode disposed at a position closer to the secondary electron detector,
The scanning electron microscope, wherein the secondary electron detector is arranged on a trajectory of secondary electrons pushed back to the sample side by the first auxiliary electrode and pulled up again by the second auxiliary electrode.   2. The scanning electron microscope according to claim 1, wherein the first auxiliary electrode has a ring shape or a U-shape that is open on a side facing the secondary detector, and the second auxiliary electrode is the first auxiliary electrode. A scanning electron microscope having a ring shape with a larger diameter or a U-shaped shape with a side facing the secondary electron detector closed.   3. The scanning electron microscope according to claim 1, wherein the potential of the first auxiliary electrode is −5 to −20V, and the potential of the second auxiliary electrode is +5 to + 20V.   The scanning electron microscope according to any one of claims 1 to 3, wherein a third auxiliary electrode having a positive potential is provided between the secondary electron detector and the sample. A sample stage for holding the sample;
An objective lens for irradiating the sample with the converged primary electron beam;
In a scanning electron microscope including a secondary electron detector for detecting secondary electrons generated from a sample by electron beam irradiation,
An annular backscattered electron detector having a primary electron beam passage hole under the objective lens;
The backscattered electron detector is disposed around the primary electron beam passage hole, and a distance from a first auxiliary electrode to which a negative voltage is applied to the optical axis of the primary electron beam is greater than that of the first auxiliary electrode. A second auxiliary electrode disposed at a position far from the secondary electron detector and to which a positive voltage is applied so as to form an electric field that pulls up the secondary electrons pushed back to the sample side by the first auxiliary electrode again And
The scanning electron microscope, wherein the secondary electron detector is arranged on a trajectory of secondary electrons pushed back to the sample side by the first auxiliary electrode and pulled up again by the second auxiliary electrode.   6. The scanning electron microscope according to claim 5, wherein the potential of the first auxiliary electrode is −5 to −20V, and the potential of the second auxiliary electrode is +5 to + 20V.   7. The scanning electron microscope according to claim 1, wherein the potential of the first auxiliary electrode can be switched to a positive potential and applied.

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